Total mesorectal excision quality in rectal cancer surgery affects local recurrence rate but not distant recurrence and survival: population-based cohort study

Abstract Background The quality of the total mesorectal excision specimen in rectal cancer surgery is assessed with a three-tier grade (mesorectal, intramesorectal and muscularis propria). This study aimed to analyse the prognostic impact of the total mesorectal excision grade on survival, and to identify risk factors for intramesorectal and muscularis propria resection in a population-based setting. Methods All patients in the Swedish Colorectal Cancer Registry with rectal cancer stage I–III ≤ 10 cm from the anal verge, diagnosed 2015–2019, undergoing total mesorectal excision were analysed. Clinical, surgical and pathological data were retrieved and analysed for the following primary outcomes: local and distant recurrence and overall and relative survival; secondary outcomes were risk factors for total mesorectal excision grading (intramesorectal or muscularis propria resection). Of note, postoperative death < 30 days or recurrence within 90 days were exclusion criteria for survival and recurrence analysis. Recurrence-free patients with less than 3 years follow-up, and patients lacking data regarding recurrence, were also excluded from recurrence analyses. Results Overall, of 7979 patients treated during the study interval, 1499 patients were eligible for recurrence, 2441 patients for survival and 2476 patients for risk-factor analyses, of which 75% were graded mesorectal, 17% intramesorectal and 8% muscularis propria. Median follow-up for survival was 42 (1–77) months. The worst total mesorectal excision grading (muscularis propria resection) was an independent risk factor for local recurrence in multivariable analysis (HR 2.73, 95% c.i. 1.07 to 7.0, P = 0.036). Total mesorectal excision grade had no impact on distant recurrence or survival. Female sex, tumour level <5 cm, abdominoperineal resection, minimally invasive surgery (laparoscopic and robotic), high blood loss, long duration of surgery and intraoperative perforation were independent risk factors for worse total mesorectal excision grading (intramesorectal and/or muscularis propria resection) in multivariable analyses. Conclusion Muscularis propria resection increases the risk of local recurrence but does not seem to affect distant recurrence or survival.


Introduction
Total mesorectal excision (TME) is the 'standard' surgical procedure for most rectal cancer patients, with a reported decrease in local recurrence rates from 25-40% to 5-10% [1][2][3] .The technique requires a precise dissection in the avascular plane between the presacral and mesorectal fascia, enabling excision of the entire rectum with surrounding lymphatic and venous drainage, preserving the mesorectal fascial envelope as an oncologic package 4 .In order to histologically assess the quality of the TME specimen, Quirke et al. defined a three-graded score, referred to as mesorectal, intramesorectal and muscularis propria, depending on what plane the dissection follows 5,6 .An association between the TME quality, according to this grading system, and local recurrence has been shown in several trials [6][7][8][9][10][11][12] .
Contradictory results have been presented regarding distant recurrence and survival in relation to TME quality 5,7,11,13 .Most studies have been retrospective with fairly small numbers of patients and many are outdated.
The primary aim of this study was to determine the prognostic value of TME quality on cancer recurrence and survival in a population-based setting.The secondary aim was to describe patient factors, tumour characteristics and treatment factors influencing the TME quality, to identify risk factors for intramesorectal and muscularis propria resections.

Study population
Data from patients diagnosed with rectal cancer between 1 January 2015 to 31 December 2019, who had undergone elective surgical intervention, were retrieved from the Swedish Colorectal Cancer Registry (SCRCR).Data included information on sex, age, body mass index (BMI), American Society of Anesthesiologists' (ASA) score, tumour level, clinical and pathological stage, perioperative and histopathological data, and local and distant recurrence.The registration of rectal cancer in BJS Open, 2024, zrae071 https://doi.org/10.1093/bjsopen/zrae071Original Article the SCRCR started in 1995 and has nearly 100% coverage 14 .The TME grade was included in the registry in 2015, which is why inclusion was set from this year.
To ensure that all included patients had undergone TME, only standard radical procedures for rectal cancer, anterior resection (AR), abdominoperineal resection (APR) or Hartmann's procedure, with a tumour level ≤10 cm from the anal verge, were included.Patients with clinical stage IV cancer, non-radical (R1) resection or missing data regarding type of surgical procedure, tumour level, clinical stage or TME grade, were excluded.Patients were divided into three groups according to TME grade (mesorectal, intramesorectal and muscularis propria), and included in risk factor analyses for intramesorectal or muscularis propria resection.The study was approved by the Swedish Ethical Review Authority (2020-00981) and complies with the guidelines of the Declaration of Helsinki.

Definitions
All included rectal cancers were adenocarcinomas.The level of the tumour was measured with rigid sigmoidoscopy, and categorized as low (0-5 cm) or middle (6-10 cm) rectal cancer.
The three-tier TME grading constructed by Quirke et al. 6 was used by pathologists when examining the rectal specimen, and registered in the SCRCR as: mesorectal plane: 'intact mesorectum with only minor irregularities of a smooth mesorectal surface, no defect deeper than 5 mm, no coning toward the distal margin of the specimen, smooth circumferential resection margin on slicing'; intramesorectal plane: 'moderate bulk to the mesorectum, but irregularity of the mesorectal surface, moderate coning of the specimen is allowed, at no site is the muscularis propria visible, with the exception of the insertion of the levator muscles'; muscularis propria plane: 'little bulk to the mesorectum with defects down onto the muscularis propria and/or a very irregular circumferential resection margin'.
TNM stage was reported according to the seventh edition of the International Union Against Cancer TNM Classification of Malignant Tumors 15 .R1 resection or non-radical resection was defined as tumour growth at the resection surface.Circumferential resection margin (CRM) was measured as the shortest distance between the tumour and the nearest edge of surgically dissected resection plane.CRM ≤1 mm was defined as positive.

Outcomes of interest
The primary outcomes were cancer recurrence and survival.Recurrences were divided into local and distant recurrences.Local recurrence was defined as an intrapelvic tumour growth (with or without distant recurrence), and distant recurrence as tumour growth outside the pelvis (with or without local recurrence).Overall survival was calculated as time from surgery to death from any cause.Relative survival was defined as the ratio of the observed survival to the expected survival in a general population, matched regarding age, sex and year of surgery.Mortality rate data was retrieved from the Human Mortality Database 16 .
Patients with postoperative death within 30 days, or cancer recurrence discovered within 90 days after surgery, were excluded from recurrence and survival analyses, as a recurrence within this timeframe indicated a disseminated disease at the time of surgery.Patients lacking data regarding recurrence were also excluded from recurrence analyses.Recurrence-free patients with less than 3 years follow-up were censored from the Cox regression recurrence analyses.Date of death was obtained from the National Cause of Death Register at the end of data collection (29 May 2021).Follow-up time was measured from date of surgery to the date of recurrence, death, censoring or data extraction.
The secondary outcome was risk factors for intramesorectal and muscularis propria resection.Age, sex, BMI, tumour level, clinical tumour stage (cT), preoperative oncological treatment, surgical approach (open or minimally invasive), estimated blood loss, duration of surgery and type of surgery (AR, APR or Hartmann's) were considered variables of interest, and analysed as potential risk factors for intramesorectal and muscularis propria resection.

Statistical methods
Continuous data are presented as median, with interquartile range (i.q.r.).Categorical data are presented as absolute numbers with percentages.The Kruskal-Wallis test and the chi 2 test were used when appropriate to test for differences in patient characteristics, pre-and perioperative data and histopathological data between the TME grades.Unadjusted and adjusted Cox regression analyses were performed, relating the TME grades to local recurrence, distant recurrence, overall survival and relative survival, presented as hazard ratios (HR) with 95% confidence intervals (c.i.).A directed acyclic graph was made to identify relevant variables and potential confounders.
The Ederer II method was used for calculations of relative survival.Overall survival and relative survival were calculated with Kaplan-Meier analyses.Differences in survival for the different TME grades were compared using the log-rank test.
Twenty-four patients who died within 30 days and 11 patients who developed cancer recurrence within 90 days of surgery were excluded in the Cox regression survival analyses, leaving 2441 patients; 1830 (75%) mesorectal, 422 (17%) intramesorectal and 189 (8%) muscularis propria.Four patients with unknown vital status due to emigration were censored in survival analysis.Median follow-up for survival was 44 (1-77) months in the mesorectal group, 39 (1-77) months in the intramesorectal group and 36 (4-75) months in the muscularis propria group.
In the Cox regression analyses for recurrence, the 136 recurrence-free patients with less than 3 years follow-up time since surgery were instead censored, leaving 1635 patients for analysis.
Patient characteristics, pre-and perioperative data are shown in Table 1.Histopathological data are shown in Table 2.There was a difference in age, BMI, tumour height, type of surgery, surgical approach, duration of surgery, perioperative blood loss, intraoperative perforation, CRM and perineural growth, when comparing the three TME groups.
Patient characteristics, pre-and perioperative data, histopathological data for 1484 patients with no TME grade (118 (3%) patients with a non-assessable TME grade and 1366 patients (34%) with missing TME grade) are shown in Supplementary material, Tables S1-S2.Patients with no TME grade were more frequently operated on with a minimally invasive technique and had more conversions to open surgery.They had a higher proportion of abdominoperineal resections and differed from patients with a TME grade in cT stage and in pathological tumour stage assessed after neoadjuvant oncological treatment ((y)pT) stage and CRM, and more frequently had perineural growth and vascular invasion compared with the patients with a TME grade.Yearly frequencies of the different TME grades are shown in Supplementary material, Table S3.
Local and distant recurrence rate at 3 years for patients with no TME grade is shown in Supplementary material, Table S4.
An additional Cox regression analysis was performed, including the patients with no TME grade.No TME grade was not associated with a higher risk of local or distant recurrence (Supplementary material, Table S5).
The directed acyclic graph made for choosing relevant variables is shown in Supplementary material, Fig. S1.
The minimal sufficient adjustments for estimating the total effect of TME grade on survival were BMI, cT stage, sex, surgical approach, tumour level and type of surgery.In addition, age and preoperative oncological treatment were added to the model.As the decision of type of surgery in the majority of cases is based on the tumour level, this variable was considered to be a collinear variable and was omitted from Cox regression analyses.

Survival
Kaplan-Meier curves for overall survival and relative survival are shown respectively in Fig. 2, Fig. S2 (Supplementary material).There was no difference in overall survival or relative survival between the three TME groups (Table 3).Overall survival rate at 3 years for patients with no TME grade is shown in Supplementary material, Table S3.An additional Cox regression analysis including the patients without a TME grade showed no association between no TME grade and worse overall survival or relative survival (Supplementary material, Table S5).

Risk factors for intramesorectal and muscularis propria grade
In multivariable analyses, APR and minimally invasive surgery were risk factors for both intramesorectal and muscularis propria resection.Female sex, tumour level < 5 cm, blood  loss > 800 ml and intraoperative perforation were risk factors for muscularis propria but not for intramesorectal resection.Duration of surgery > 9 h was a risk factor for intramesorectal but not for muscularis propria resection (Table 4).

Discussion
In this national population-based cohort study, patients with TME grade muscularis propria had a higher risk of developing local recurrence, but the TME grade had no impact on distant recurrence, overall survival or relative survival.The association between TME grade and local recurrence rate is in line with several earlier studies [6][7][8][9][10][11] .While there are studies showing no correlation between local recurrence and TME grade, these are small or have a relatively short follow-up 5,13,17,18 .The lack of correlation between distant recurrence or overall survival and TME grade is consistent with two recent studies 10,11 ; one of them, however, found mesorectal resections to correlate with increased 5 year disease-free survival 11 .Muscularis propria grade has been shown to predict distant recurrence, disease-free survival and overall survival with a two-tier grading, combining mesorectal and intramesorectal versus muscularis propria 13 .Two-tier grades, combining intramesorectal with either mesorectal or muscularis propria grade, were used in some earlier studies.In this population-based study the number of patients was considered sufficient for analysing the three grades separately.Several factors were associated with intramesorectal or muscularis propria grade dissection.Low tumour height (< 5 cm) was associated with muscularis propria resection, and APR, the procedure of choice in low tumours, was related to both intramesorectal and muscularis propria grade.The perineal part of an APR is challenging, since no well-defined dissection plane is offered, and a higher incidence of intramesorectal and muscularis propria grade has been reported [5][6][7][8]10,12,17,19 . A higher rae of CRM involvement, perforation, local recurrence and poorer overall survival have been described in APR compared with AR 20,21 .
Minimally invasive surgery was associated with intramesorectal and muscularis propria grade.Two meta-analyses and two randomized controlled trials have reported a lower incidence of complete TME resections with minimally invasive surgery [22][23][24][25] , whereas some studies have found no association between TME quality and surgical approach 19,26,27 .A recent study comparing robotic and open mesorectal excision reported similar oncologic outcomes 28 .In 2015, robotic surgery was being introduced in many parts of Sweden, and the finding of more intramesorectal or muscularis propria resections could be related to the learning curve.The increasing number of intramesorectal and muscularis propria grading during the study interval is perhaps due to a more careful examination by pathologists, as the awareness of TME grade increased from 2015, when introduced.
The difficulties of rectal cancer surgery in men are often stressed, whereas rectal surgery in females is generally considered easier, partly due to their wider pelvis.In this study, however, female sex was a risk factor for muscularis propria resection, a finding supported by one previous study 17 .The reason for this finding is unclear, but may reflect higher attentiveness during the mesorectal dissection in men, since it is considered more difficult.
A long duration of surgery, intraoperative perforation and high blood loss were all associated with intramesorectal and/or muscularis propria grade.All these risk factors probably indicate an advanced tumour, or strenuous surgery, with a following increased risk of not achieving mesorectal resection.
This study has a few limitations.Thirty-seven per cent of the cases in this study had no TME grade reported by the local pathologists.While this weakens the study, no TME grade was not associated with a higher local or distant recurrence rate or worse overall or relative survival, thus the reason for not grading did not seem to correlate with difficulty grading an inferior specimen.Fewer missing TME grades would have been a strength but since the study cohort was large and there were no obvious signs of selection bias, the results are considered reliable.Also, since this was a retrospective register-based study, no validation of the pathologists' TME grade could be performed.Thirty per cent of the patients had no registered 3 year follow-up, mainly due to shorter follow-up at SCRCR data extraction.In Sweden, rectal cancer patients routinely undergo computer tomography of the chest and abdomen, and outpatient visit (including clinical examination and rigid sigmoidoscopy when possible), at 1 and 3 years postresection surgery.The SCRCR registers events at the 3 year and 5 year follow-up.Recurrences discovered earlier can be registered separately, but are frequently registered a posteriori in conjunction with the 3 year or 5 year follow-up.Therefore, this study could underreport recurrences.The follow-up time could also have been too short to detect any differences in distant recurrence or survival.The proportion of patients with an intraoperative tumour perforation was higher in the muscularis propria group.However, none of the patients with a tumour-close perforation developed local recurrence in this study, even though perforation is an established risk factor for local recurrence.
While perineural growth was higher in the muscularis propria group, when excluding patients with perineural growth from analyses, the local recurrence rate was still higher in the muscularis propria group, indicating that muscularis propria grade is an independent risk factor for local recurrence.In addition, multivariable analysis of local recurrence was performed despite few events (8 variables and 43 events), as the rule of 10 events per variable might be too strict and five events per variable may give equal results 29 .
The findings reported here stress the importance of good surgical quality and underline TME grade as a measurement of that.Tumour level, female sex, blood loss and intraoperative perforation were risk factors for muscularis propria resection, and APR and minimally invasive surgery were risk factors for both intramesorectal and muscularis propria resection.Extra caution is warranted in these patients.

Overall survival in patients with mesorectal, intramesorectal and muscularis propria resection Table 4 Risk factors for intramesorectal or muscularis propria grade, results from logistic multiple regression models
Numbers in bold are statistically significant.OR, odds ratio; c.i., confidence interval; BMI, body mass index; c, clinical; RT, radiotherapy; CT, chemotherapy; CRT, chemoradiotherapy; AR, anterior resection; APR, abdominoperineal resection.